Television broadcasts are transmitted using high frequency RF carriers. These high frequency carriers are easily deflected, reflected and blocked by obstructions such as concrete walls of buildings, trees and mountains. As such, the strength of television broadcast signals varies greatly from one area to another. The quality of the reception of a given television station depends on many factors, including the location of the television receiver, the position of the television receiver, the quality and type (indoor or outdoor) of the receiving antenna used and the quality of the antenna cable used. The shielding effect of the antenna cable used will also have a substantial effect on the quality of a received television signal.
Generally, in an area where the signal of a particular television station is considerably weak, the use of a television signal booster (RF amplifier) to increase the signal strength for that particular television station is very favourable. However, as signal strengths from other television stations are often already more than sufficient, it may be necessary to turn the television signal booster off when the television receiver is receiving program from a television station with a strong signal. Otherwise, the received RF signal will be over-amplified, causing the signal to saturate in the receiver's amplifier, adding more noise to the original signal.
If a television station broadcasts at a frequency sandwiched between two stronger television stations, turning on the booster while receiving signal from that station may result in signals from the adjacent television stations being amplified, causing interference and adding noise to the original signal.
A cable television service is normally characterized by having a much larger number of stations than available through an off-the-air terrestrial service, and accordingly by an overall narrower separation of neighboring channels. Generally, the RF signals of a cable television service will be of a desirable signal strength and will not require boosting. Indeed, switching on a booster amplifier when connected to a cable television service most often causes interference by adjacent channels being amplified together and causing a noticeable beat effect on the TV picture. This effect is manifest as, for example, wide diagonal or wavy beat lines spoiling the picture.
Conventional television receivers incorporating an internal television signal booster require the user to either switch on or off the booster manually, depending on the condition of a received television signal. In fact, the user will most likely keep the booster amplifier permanently switched on. Moreover, the lay user cannot be expected to understand that the booster can worsen his TV picture if it is switched on when not required, such as when a cable TV service is received.
Japanese patent application publication 2001-244836 discloses reducing interference due to beat noise components, by lowering the gain of the high frequency amplification during channel search. This is achieved by lowering the AGC signal feedback to the RF amplifier during automatic channel search.
Japanese utility model H05-20472 teaches disabling the operation of the RF amplifier during channel preset, to avoid cross modulation and hum modulation.
Japanese patent application publication H05-304640 discloses bypassing an RF amplifier when the AGC voltage satisfies a predetermined voltage.
Japanese utility model S58-138465 discloses turning an RF signal booster on or off depending on the strength of the input signal.